1. Technical Field
The present invention relates to a communication system, a receiver, and an adaptive equalizer. More particularly, the present invention relates to an adaptive equalizer which compensates signal degradation caused by a transmission path or the like.
2. Related Art
In the serial communication and other fields, an equalizer is used which compensates signal degradation caused by a transmission path. The equalizer is a circuit for compensating degradation of a transmission signal in terms of amplitude and timing which is caused by, for example, the amplitude and delay characteristics of the transmission path, as disclosed in, for example, Unexamined Japanese Patent Application Publication No. 2005-292004.
When transmitted out from a transmitter, a data signal may have a large eye opening at the output end of the transmitter. However, the data signal may have a small eye opening at the input end of a receiver after transmitted through a transmission path. For example, when the transmission path has low pass filter characteristics, the higher-frequency components of the data signal experience a larger decrease in amplitude, which changes the phase of the data signal. Therefore, the waveform of the data signal which is observed at the input end of the receiver is deformed. To deal with this issue, an equalizer which has predetermined filter characteristics is utilized. The equalizer can make compensation for the amplitude and delay characteristics of the transmission path. As a result, the data signal input into the receiver can have a large eye opening.
Here, each transmission path has different amplitude and delay characteristics. This means that the filter characteristics of the equalizer can not be uniquely determined. To deal with this issue, an adaptive equalizer which is capable of automatically adjusting the filter characteristics thereof may be utilized.
FIG. 13 illustrates an exemplary configuration of an adaptive equalizer 200. The adaptive equalizer 200 includes therein a filter 210, a level comparing section 220, a signal generating section 230, a subtracting section 240, a switch 260, and an adjusting section 250.
The filter 210 has predetermined filter characteristics. The filter 210 passes a received input signal therethrough. The adjusting section 250 adjusts the filter characteristic of the filter 210. For example, the adjusting section 250 causes a transmitter to transmit a known input signal to the filter 210. Here, the adjusting section 250 causes the signal generating section 230 to generate the same signal as the known input signal.
The subtracting section 240 receives the signal generated by the signal generating section 230, and the input signal which has passed through the filter 210. The subtracting section 240 obtains the difference between these two signals, and inputs the obtained difference into the adjusting section 250.
The adjusting section 250 adjusts the characteristics of the filter 210 so that the value of the difference received from the subtracting section 240 becomes equal to substantially zero. In other words, the adjusting section 250 adjusts the characteristics of the filter 210 in such a manner that the signal which has passed through the filter 210 becomes substantially equivalent to the known input signal. In this way, the adaptive equalizer 200 can compensate the signal degradation caused by the transmission path.
By performing the above-described adjusting procedure, for example, at the start-up of a communication system, the characteristics of the filter 210 can be adjusted in advance in accordance with the characteristics of the transmission path. Also, while the communication system is actually operating, the characteristics of the filter 210 can be adjusted in accordance with the change in the amplitude and delay characteristics of the transmission path which is caused by, for example, a change in the temperature, in such a manner that the switch 260 causes the signal output from the level comparing section 220 to be input into the subtracting section 240.
Here, the adaptive equalizer 200 shown in FIG. 13 requires the signal generating section 230 which generates the known input signal. Also, since the two signals input into the subtracting section 240 are transferred through different paths, there is a difference between the phases of the two signals input into the subtracting section 240. Therefore, the adjusting section 250 inevitably sets the characteristics of the filter 210 based on the value including the error corresponding to the difference between the phases.